Wide-angle lenses for capturing a wide field of view are used in the field of on-vehicle cameras and surveillance cameras. In addition, progress in image processing techniques has made it possible to correct distortion of images in real time, and there have been increasing demands for super-wide-angle lenses having a still wider field of view. In particular, in accordance with the problems of installation space, increase in the resolution of picture elements, and applications in various fields, there have been demands for small, high-resolution, and low-cost optical apparatuses.
Such an optical apparatus is required to satisfy the following requirements:                low cost (small number of lenses, use of resin, formability, assemblability)        bright (small Fno)        small (overall length, lens diameter)        wide field of view (a large angle of view that exceeds 180 degrees depending on the case)        high resolution (suitable correction of basic aberrations)        projection method selected as necessary (central projection, conformal projection, stereographic projection, etc.)        
If an optical apparatus with a small number of elements that satisfies the above-described requirements can be obtained, such an optical apparatus can be applied to various fields.
FIG. 16 shows an optical apparatus described in Patent Document 1 mentioned below. This optical apparatus is a basic wide-angle lens in which the angle of field is increased by a negative lens positioned at an object side and an image is formed on an image pickup device by a positive lens. This optical apparatus is constituted by a minimum number of lenses, and has a two-lens structure including a negative lens and a positive lens. However, the two-lens structure has, of course, a limited degree of design freedom, and it is difficult to increase the resolution or set an arbitrary projection method. Therefore, the application of this optical apparatus is limited.
FIG. 17 shows an optical apparatus described in Patent Document 2 mentioned below. This optical apparatus is an example of a typical wide-angle lens in which a plurality of concave lenses are disposed on the object side of an aperture stop and concave, convex, and convex are disposed on an image side of the aperture stop. In this optical apparatus, aberrations are corrected and a projection method is set using multiple lenses. Therefore, although there is a relatively large degree of design freedom, it is difficult to reduce the number of lenses because the basic structure of the lens unit on the image side of the aperture stop is determined.
FIG. 18 shows an optical apparatus described in Patent Document 3. This optical apparatus includes a concave lens, a meniscus lens, and a double-convex lens arranged in that order from the object side, and is characterized in that the second lens from the object side is a thick negative meniscus lens with a small Abbe number. The lateral chromatic aberration can be effectively corrected by suitably setting the positions of the thick meniscus lens with a small Abbe number and an aperture stop. However, the angle of view is 76°, which is relatively small, and an image with a large field of view cannot be formed.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 02-077712
Patent Document 2: Japanese Unexamined Patent Application Publication No. 14-072085
Patent Document 3: PCT Japanese Translation Patent Publication No. 09-503600